1. Field of the Invention
The present invention relates to a liquid crystal composition for an electrooptic element, wherein the liquid crystal composition contains a pleochroic anthraquinone dye, and to a liquid crystal device employing such a composition. The invention also extends to the dyes.
2. Description of the Prior Art
A pleochroic dye is a dye whose absorption properties vary according to the incident angle of the light relative to the molecular alignment. The absorption transition moment of a pleochroic dye having parallel dichromatic characteristics is substantially parallel to the direction of the longitudinal axis of the molecules, so that this dye absorbs little light passing along its longitudinal axis but has a maximum absorption of light passing in the direction perpendicular to its longitudinal axis.
As the pleochroic dye molecules, when placed in a liquid crystal material, adopt an orientation which follows the orientation of the liquid crystal material, one can change the direction of the orientation of the dye molecules by changing the direction of the orientation of the liquid crystal material by applying the appropriate electric field.
This phenomenon is known as the "guest-host effect" in which the pleochroic dye is regarded as "guest" and the liquid crystal material as "host".
By utilizing this phenomenon a color display device which is electrically controlled can be designed. For example, when interposed between opposed glass plates which carry transparent electrodes and whose inner surfaces have been subjected to a homogeneous orientation treatment, liquid crystal molecules having positive dielectric anistropy and including a pleochroic dye having a parallel dichromatic characteristic exhibit homogeneous orientation, i.e. the longitudinal axis of the liquid crystal molecules in parallel to the glass plates, and at the same time the pleochroic dye molecules adapt as same orientation as the liquid crystal molecules. When white light passes through the liquid crystal material including a pleochroic dye in this state, a specific wavelength region of the white light is absorbed by the dye. The crystal appears colored.
If an electric field is applied to the transparent electrodes, the liquid crystal molecules turn to a homeotropic orientation in which the longitudinal axis of the molecules is perpendicular to the glass plates and the longitudinal axis of the dye molecules also turn to perpendicular to the glass plate. In this state, a white light which passes perpendicularly to the plates is little absorbed by the pleochroic dye because the electric vector of the incident light is perpendicular to the longitudinal axis of the dye. This means that the liquid crystal material is seen as transparent.
Accordingly it is possible to display information selectively utilizing the difference between the colored state and the transparent state.
The guest-host effect described above may be also obtained by utilising a nematic liquid crystal which is caused to have a special structure by addition of an optically active substance, a cholesteric liquid crystal, or a smetic liquid crystal. Color display systems utilizing these liquid crystal materials are described in detail in Japanese Laid Open Patent Applications Nos. 127645/74 and 153053/79.
In order to provide a good quality color display utilizing the guest-host effect, it is important that the pleochroic dye in the liquid crystal molecules should satisfy the following requirements.
First, it is necessary to provide maximum contrast between the colored state and the transparent state, i.e. the regularity of orientation of the pleochroic dye molecule relative to the orientation of the liquid crystal molecules should be high. This is however achieved only to a limited degree because of random thermal disturbance.
The regularity of orientation of the pleochroic dye molecules in the liquid crystal molecules is measured by a quantity known as the order parameter S which is defined by the following formula: ##EQU1## wherein cos.sup.2 .theta. is a time-averaged term and .theta. is the angle formed between the absorption transition moment vector of the pleochroic dye molecule and the orientation of the liquid crystal molecule.
The order parameter S is theoretically in the range -0.5 to 1. The closer to 1 it is, the higher is the regularity of orientation. Accordingly when a pleochroic dye having an order parameter as near one as possible is used, it is possible to obtain a good quality color display.
Second, since it is necessary to produce a clear color, the solubility of the pleochroic dye in the liquid crystal material should be sufficiently high that a desirable concentration can be easily obtained. Dye concentration is usually selected according to the thickness of the liquid crystal layer of the element and the absorptivity coefficient of the dye.
In addition to the above described requirements, it is generally required that the stability of the dye against light, heat, water and especially against oxygen should be high.
There are many known dyes, for example merocyanine type dyes, azo type dyes, azomethine type dyes, and anthraquinone type dyes, which are conventionally used as pleochroic dye for guest-host type liquid crystal display device, but only limited numbers of dyes are found to be suitable in liquid crystal applications because most of these dyes are considered to satisfy only some of the requirements explained above.
European Patent Application No. 0002104 discloses many anthraquinone dyes with anilino substituents e.g. ##STR1## The solubilities of most of these dyes are not given. One of the dyes will be referred to again below.